1. Field of the Invention
This invention resides in the technology of ceramic dielectric materials, and particularly barium titanate.
2. Description of the Prior Art
Barium titanate (BaTiO3) is a ferroelectric material whose high permittivity at room temperature has made it a material of interest in the manufacture of thermistors, multilayer ceramic capacitors (MLCCs), electro-optic devices, and as a capacitor material in down-sized portable machines and dynamic random access memories (DRAMs). With the ever higher temperatures and voltages to which these devices are exposed and the continuing demand for smaller and lighter components, there is an ever increasing need for barium titanate materials that perform reliably, that store energy at a high density, and that maintain a high capacitance.
One of the features of barium titanate that has been investigated for its influence on permittivity is the grain size. The results of these investigations have been reported in the literature, examples of which are Ota, T., J. Takahashi, and I. Yamai, “Effect of microstructure on the dielectric property of ceramics,” Key Engineering Materials, Volumes 66 and 67 (1995), pages 185-246; and Takeuchi, T., Y. Suyama, D. Sinclair, and H. Kageyama, “Spark-plasma-sintering of fine BaTiO3 powder prepared by a sol-crystal method,” J. Mater. Sci. 36 (2001), pages 2329-2334. According to these reports, the dielectric constant of barium titanate is independent of the grain size for grain sizes greater than 2 microns, where the value of the dielectric constant at 25° C. is about 2,000. For smaller grain sizes, i.e., within the range of 0.7 micron to 2 microns, the dielectric constant at 25° C. rises above 2,000 to a maximum of about 5,000 at a grain size of approximately 1.0 micron. For grain sizes below 0.7 micron, the dielectric constant is again lower, decreasing with decreasing grain size.
Dielectric constants higher than 2,000 have been achieved by the use of spark plasma sintering to densify the barium titanate compacts. Two studies by Takeuchi report dielectric constants of about 7,000 obtained in this manner. Takeuchi, T., Y. Suyama, D. Sinclair, and H. Kageyama, “Spark-plasma-sintering of fine BaTiO3 powder prepared by a sol-crystal method,” J. Mater. Sci. 36 (2001), pages 2329-2334; Takeuchi, T., M. Tabuchi, and H. Kageyama, “Preparation of dense BaTiO3 ceramics with submicrometer grains by spark plasma sintering,” J. Am. Chem. Soc. 82 [4] 939-943 (1999). In both cases, the density of the sintered product was about 98% of theoretical density. Spark plasma sintering is a means of densification, which might suggest that further densification would lead to an even higher dielectric constant. While density is in some cases related to grain size, the findings of Ota et al. (referenced above), however, suggest that decreasing the grain size would be expected to lower the dielectric constant rather than to raise it.